Note that the Bulb mode over USB triggering not being supported is an issue up to the D5300 (Some people said D5200 works, I know the D5100 doesn't work) and D7200 (D7100?) and impossible (Nikon left it out) on D3xxx series. I'm not sure on the more expensive cameras.
A trick I got introduced to if you have an IR port is NikonRemote ( github.com/outofjungle/NikonRemote ) Which you can feed into INDI's serial port and it works. Which uses just a Uno, resistor and IR LED. (It may not work with clones as is because it relies on the reset behavior of the Uno, but if you are relying on an external thing (or gpio like from a Pi), you can trigger it by the arduino's reset pin.)